The three peak emission system is known as one of the methods for improving color rendering properties of a fluorescent lamp without impairing its efficacy. In other words, it is known to use three types of phosphors which have light emission spectra with peaks in the vicinities of 450 nm, 540 nm and 610 nm, respectively, and with relatively narrow half value width.
Phosphors which may be conveniently used by the above-mentioned three peak emission system may include a europium activated divalent strontium.calcium chlorophosphate phosphor as a blue-emitting phosphor having a peak of the spectrum in the vicinity of 450 nm, a cerium and terbium activated yttrium silicate phosphor as a green-emitting phosphor having a peak of the spectrum in the vicinity of 540 nm, and a europium activated yttrium oxide phosphor as a red-emitting phosphor having a peak of the spectrum in the vicinity of 610 nm.
When the lamp ambient temperature becomes low, the starting voltage of the lamp generally becomes high. Therefore, when a lamp is used in a cold area or at a position where it is subject to cool temperatures, the starting voltage is an important problem. When the starting voltage becomes higher than the commercial supply voltage, the lamp cannot be turned on. In a general fluorescent lamp, the practical starting voltage must be a voltage about 10% lower than the commercial supply voltage for the purpose of preventing non-starting of the lamp if the lamp ambient temperature is 21.degree. C. Therefore, when the commercial voltage is 100 V, the practical starting voltage must be 90 V or lower.
In a fluorescent lamp of the three wavelength method which uses as blue, green and red-emitting phosphors a europium activated divalent strontium.calcium chlorophosphate, a cerium and terbium activated yttrium silicate phosphor, and a europium activated yttrium oxide phosphor, the starting voltage becomes high and frequently exceeds 90 V, thus limiting the installation location of the lamp.